1. Field of the Invention
The present invention relates to the transmission of multimedia broadcast multicast services (MBMS) radio bearer (RB) configuration information. In particular, the present invention relates to the transmission of such information to enable user equipment (UE) to receive a point-to-multipoint (PTM) multimedia session.
2. Description of the Related Art
The term “MBMS” represents a service that provides users of a cellular telecommunications system with information, such as the streaming of video and audio files, news updates, or the like. The service may be one single ongoing session, or comprises a number of successive individual sessions during which part of the data is transferred. The MBMS may be utilized in broadcast mode wherein information is broadcast to all users within a cell. Alternatively, a multicast mode may be used wherein a subgroup of users within the cell are sent information they alone have requested, for example, football results. The multicast mode is useful for services that require a subscription.
It is planned to use MBMS with Universal Mobile Telecommunications System (UMTS) networks. Several 3rd Generation Partnership Project (3GPP) technical specifications are available for providing information on the relevant specifications. The technical specifications can be found at http://www.3gpp.org/specs/specs.htm, the entire content of which is hereby incorporated by reference. Some examples relevant to this patent application are 3GPP TS 22.146, 3GPP TS 25.301, 3GPP TS 25.346, 3GPP TS 25.401 and 3GPP TS 25.246, the entire contents of all of which are incorporated by reference.
A conventional UMTS radio network architecture is shown in FIG. 1. A core network 101 is connected to one or more radio network subsystems 103 via the Iu interface. Within each radio network subsystem 103, a radio network controller 105 connects to one or more base stations via the lub interface. In the case of Universal Terrestrial Radio Access Network (UTRAN), Node B 107 is the base station. Each radio network controller 105 may be interconnected to other radio network controllers 105 by using the Iur interface.
Each Node B 107 can control one or more cells. Within each cell, there are numerous UEs some of which are able to receive MBMS services provided by a Broadcast Multicast Service Centre (BM-SC) located in the core network 101. The MBMS services are either broadcast to all UEs within the cell or multicast to only a selected group of UEs that have subscribed to the particular service. The UEs are mobile communication devices, for example, mobile telephones and PDAs. The UE connects to the Node B 107 via a radio interface (Uu).
The type of transfer mode to be used for a data transfer session of a specific MBMS service, i.e. point-to-point (p-t-p) or point-to-multipoint (PTM), is chosen depending upon the number of UEs within the cell that are interested in receiving the particular service and/or session. The UTRAN decides which transfer mode is used. For example, if a limited number of UEs are within the cell (for example, less than four) the UTRAN makes a decision to transfer the information supplied by the MBMS over the radio interface using the p-t-p mode in order to consume a minimum amount of radio capacity. Alternatively, if more UEs are within the cell, the PTM transfer mode is used to utilise the available radio capacity more economically.
When the UTRAN makes the decision to transmit information supplied by the MBMS in PTM mode, radio bearer configuration information is provided within the MBMS control information. MBMS control information is transmitted on the MBMS control channel, known as the multicast control channel (MCCH) over the radio interface (Uu) to the UE. The radio bearer configuration information provided on MCCH in the current cell allows a UE to receive services provided in PTM mode that are available in the current cell and potentially available from neighbouring cells. For these services, the information provided comprises the radio bearer configuration required for the UE to receive the MBMS data from the radio bearer in the current cell, as well as from the radio bearer in some neighbouring cells that are able to provide the same MBMS data PTM over a Multicast-Traffic-Channel (MTCH). The UE can combine the MBMS data received from the neighbouring cell radio bearer with the MBMS data received from the current cell radio bearer in order to improve the reception quality. Reception quality is improved by reducing the power required in each cell to provide sufficient coverage. The signalling of the radio bearer configuration information is discussed in more detail below. That is, the UE can receive a time aligned MTCH transmission from a neighbouring cell without receiving the MCCH of that cell.
The RB configuration information associated with each service signalled on the MCCH is repeated a number of times with exactly the same content. This MCCH information is transmitted at the beginning of every repetition period within a set modification period. Referring to FIG. 2, which shows scheduling of messages on the MCCH according to conventional systems, any messages that contain service specific MBMS RB information 201 that has changed since the last modification period are placed at the start of the repetition period, while any other messages containing unchanged service specific MBMS RB information 203 or unchanged Common RB information 205 are placed at a later time slot in the repetition period. That is, an MBMS notification is first transmitted giving information on each changed service. Subsequently information is transmitted in the form of one or more MBMS service PTM RB information messages or common RB information messages, comprising any changed RB configuration information associated with the MBMS services. The UTRAN then provides scheduling information indicating the final Transmission Time Interval (TTI) containing the changed information so that UEs that have previously received the RB configuration information are not required to read any further RB configuration information after the indicated TTI.
Each service provided by the MBMS typically corresponds with one radio bearer. The radio bearers are mapped onto a transport channel, Forward Access Channel (FACH) in the case of MBMS. One or more radio bearers are mapped to a single common FACH. The FACH provides the means to transfer data with a certain quality of service (QoS), for example, a certain data rate, residual error rate and delay. Therefore, any services that require the same quality of service are typically mapped to the same transport channel (FACH), whereas services that require lower or higher quality of services are mapped to other transport channels (FACH).
One or more transport channels (FACH) are mapped to a physical channel. In the case of MBMS, the secondary-common control physical channel (S-CCPCH). Each physical channel has certain characteristics, for example, type of power control, cell coverage, use of certain techniques, such as transmit diversity.
In the conventional UMTS, signalling of the MBMS radio bearer configuration information is performed by using two messages.
First, a “common RB info” message is used to hold and transmit RB configuration information that is either common between two or more services, or common between two or more cells.
This “common RB info” message may comprise either changed or unchanged information depending on whether or not the configurations of the radio bearers have changed since the previous modification period.
Second, for every service that is being provided using the PTM mode, a separate “service RB info” message is transmitted in order to specify how the radio bearer corresponding to the service is configured in the cell the UE is currently in, and also how the radio bearer is configured in any neighbouring cells that also provide the service over the MTCH of the neighbouring cell. This information provides the necessary configuration details for a UE to be able to receive one particular service.
Configuration information in the “common RB info” message can be used for any common configurations (either between cells or services). In other words, if there is configuration information common between services or cells, it would be transmitted in the “common RB info” message. The common configuration information is then not required in the “service RB info” message. The “service RB info” message is merely required to comprise a reference to the applicable common configurations within the “common RB info” message.
For each service available within a cell, information on the entire radio configuration is provided. This radio configuration information comprises the radio bearer configurations of the actual radio bearers (“real” RB configurations), the transport channel (TrCh) configurations (FACH for MBMS) and physical channel (PhyCh) configurations (S-CCPCH for MBMS). Therefore, if either the “real” RB configurations or transport channel configurations are common between a number of services available in that cell, references to these common configurations are duplicated, thus causing increases in the signalling overhead on the MCCH. For example, if one MBMS service is provided by a first radio bearer, and a second MBMS service is provided by a second radio bearer, where the services require the same quality of service (QoS), the two services will be mapped to the same transport channel (FACH). As there will be a separate “service RB info” message for each service, references to the RB common configurations for the FACH within each of the “service RB info” messages will be duplicated.
The duplication will be illustrated in greater detail. Assuming 5 services are provided in the current cell using the PTM transfer mode and assuming that for each service the PTM radio bearer configuration is provided for 6 neighbouring cells also, with the current state of the art, for each service a message is provided as shown in table 1.
TABLE 1“Service RB Info” message - for service ATrChPhyChactual RB-commoncommonCellcommon configconfigconfigCurrent111N-1111N-2111N-3121N-4121N-5231N-6112
Table 1 depicts information that is transmitted in the ‘Service RB info’ message for one particular MBMS service. Inside this message are references to RB, TrCh and PhyCh common configuration information that is transmitted within the ‘Common RB info’ message. The ‘Service RB info’ message comprises the configuration information for the current cell and all neighbouring cells (six in this example) for one particular MBMS service.
The first column indicates which cell the data is referring to, either the current cell, or one of the six neighbouring cells (N-1 through N-6). The numbers in the next three columns indicate the types of RB, TrCh and PhyCh configuration used in the relevant cell. Each of these numbers refers to a specific type of configuration as included in the “common RB info” message. The service A shown in the example covered by table 1, applies actual RB configuration type 1, TrCh configuration type 1 and PhyCh configuration type 1 in the current cell. The table also shows that, for this example, the common RB configuration comprises at least 2 actual RB configurations, 3 TrCh configurations and 2 PhyCh configurations.
The case where all MBMS services in a cell are mapped to the same physical channel can be regarded as typical. The current state of the art, which involves a separate message per service, therefore has a reference to indicate the PhyCh configuration used in the concerned cell for each service. In the example case where five services are provided, there will be a reference to PhyCh configuration 1 indicated five times, i.e. one for each service. This produces a large amount of signalling, which merely provides the same information to the UE a number of times. Further, where it is chosen to comprise actual configuration information rather than references, the duplication of this configuration information substantially increases the signalling overhead.
When specifying the RB configuration per service, the UE is not required to read RB configuration information concerning services that it is not interested in i.e. non-active services. This helps to reduce the time the UE needs to read the MCCH and so reduces the UE power consumption. However, this comes at the cost of duplication (whether the information is comprised of actual configuration information or references to actual configuration information) of MBMS control information on MCCH as shown in the above example.
Thus, two conflicting requirements appear within the configuration of the conventional system. That is, in order for UE power consumption to be kept to a minimum, information needs to be duplicated in order to reduce the time required for the UE to acquire the MBMS control information on the MCCH. However, the duplication of information increases the signalling overhead. That is, the amount of MBMS control information is not kept within reasonable limits when compared with the MBMS data traffic.